1. Field of the Invention
The present invention relates to an air cleaning apparatus for cleaning air of an atmosphere within and outside processing units of various processing apparatuses.
2. Description of the Related Art
In order to process an object in a desired state without adhesion of impurities, it is necessary to clean the atmosphere within and outside a processing unit in which an object is processed. For example, a film-forming apparatus such as a CVD apparatus, an oxidizing apparatus or a diffusion apparatus for forming a thin film on a surface of a semiconductor wafer in a semiconductor device manufacturing process is provided with an air cleaning apparatus for removing through a filter impurities present in an atmosphere within a processing unit in which a semiconductor wafer is transferred for processing. Thereby, adhesion of impurities on the semiconductor wafer is prevented, and a process of exactly forming a thin film on a semiconductor wafer can be performed. Such an air cleaning apparatus as applied to the CVD apparatus will now be described in detail.
A CVD apparatus, in particular a vertical CVD apparatus, comprises in general a cylindrical processing unit in which a film is formed on a semiconductor wafer, a wafer boat for holding a semiconductor wafer within the processing unit, a convey mechanism for supporting the wafer boat and inserting and taking out the wafer boat into and from the processing unit, a transfer mechanism for transferring the semiconductor wafer onto the wafer boat supported by the convey mechanism, and a housing having a space in which these components are arranged.
For example, an air blow fan and a filter for removing dust such as particles are provided within the housing, in order to prevent particles from adhering to the semiconductor which is moved within the housing by the transfer mechanism or along with the wafer boat. In this case, an air stream is formed by the air blow fan so as to flow from the rear to the front of the housing. By situating the filter along the flow of the air stream, a clean air stream is always formed within the housing, and adhesion of particles to the semiconductor wafer is prevented.
Recently, the thickness of a film layer, such as a silicon oxide film or a silicon nitride film, formed on a semiconductor has decreased more and more with an increase in diameter of the semiconductor wafer and very fine processing of the wafer. Accordingly, strict control of physical properties such as electrical characteristics of the formed layer is required, and a very slight amount of impurities in the atmosphere for processing influences the finished state of the formed layer greatly. On the other hand, in fine processing of 16 MDRAM or a higher level, fine particles, which cannot be explained from an aspect of a dust adhesion phenomenon, appear on the formed layer. Physical properties of the formed layer, such as electrical characteristics, are degraded due to those particles, resulting in a lower yield.
In order to solve this problem, the inventors studied and analyzed in detail impurities which will become a factor of degradation in physical properties such as electrical characteristics of the formed layer, and, in particular, components of a slight amount of gas in the atmosphere for processing, by means of modern analyzing technology. The inventors found that the components of such a slight amount of gas are a factor of the particle generation phenomenon. In addition, a qualitative analysis was conducted on the components of a slight amount of impurity gas contained in the air within the housing of the CVD apparatus. As a result, it was found that the air within the housing contains a small amount of gas, e.g. an organic compound such as hydrocarbon, a phosphide, a boride, etc.
The inventors further studied the source of the small amount of impurity gas. As a result, it was assumed that the source of impurity gas is the material of the dust-removing filter and a small amount of compound present in the material is liberated within the housing in the form of impurity gas. The components of the compound remaining in the filter material is compared with the components of the impurity gas specified by the aforementioned gas analysis, and it turned out that both components coincide. A high-performance air filter made of HEPA, ULPA, etc. is generally used as dust-removing filter. The air filter is fixed to the filter frame by means of an organic adhesive. The filter frame is made of a metallic material. The air filter is formed in a sheet-like shape of a mixture of SiO.sub.2 as a main component and other various components such as boron oxide, etc. Specifically, the organic impurity of the adhesive for coupling the air filter and filter frame, as well as the small amount of the compound remaining in the material of the dust-removing filter, is liberated within the housing as impurity gas.
It was also found that a substance which facilitates liberation of a boron compound, etc. from the dust-removing filter flows into the CVD apparatus from the clean room. Specifically, such a small amount of impurity gas is liberated from the dust-removing filter (the liberation is sometimes facilitated by a substance flowing into the CVD apparatus from the clean room), and the impurity gas is conveyed to the processing atmosphere by the air blow fan situated within the housing. The impurity gas adheres to a film layer on the semiconductor wafer in the film forming process, and fine particles are formed on the surface of the film layer. In particular, when the wafer boat is taken out of the processing unit by the convey mechanism after the film forming process, the temperature within the housing increases considerably by radiation heat from the wafer boat. Consequently, the temperature of the dust-removing filter rises and the liberation of impurity gas from the dust-removing filter is facilitated. In addition, the film formed by the film formation process is chemically active at high temperatures. Because of these two factors, fine particles are easily formed on the surface of the formed film.